Slope winds result from the diurnal cycle of heating and cooling of the planetary boundary layer along elevated terrain. Daytime anabatic winds flow upslope, whereas at night, katabatic winds flow downslope.
Diurnal mountain winds are generated by insolation of the planetary boundary layer during the daytime and longwave emission-driven cooling during nighttime. This heating(cooling) produces horizontal temperature gradients between air above heated (cooled) topography and the air in valleys (Barry 2008). These turbulent flows represent the lower branch of a closed thermally direct circulation resulting from inclined heated or cooled planetary boundary layers that form above topography in a stratified atmosphere (Shapiro and Fedorovich 2008). Due to this thermal forcing, these winds are also commonly referred to as thermally driven winds. Diurnal mountain winds have been long studied (see Barry (2008) and Zardi and Whiteman (2012) for reviews) due to their importance in mountain environments with regards to day-to-day weather, climatological characteristics, transport of water vapor and pollutants, triggering of moist convection, agriculture, buildings, and biodiversity.
The diurnal mountain wind system is composed of three interacting wind systems occurring at multiple scales. In growing order of spatial scale, these include the slope, valley, and mountain range (Whiteman 2000). The diurnal mountain wind system is characterized by lower wind speeds (typically 1–6 m/s) and thus easily overwhelmed by synoptic-scale winds. These winds tend only to be observed during relatively quiescent synoptic weather conditions characterized by weak, anticyclonic flow aloft (Smith and Skyllingstad 2005; Stull 2017) with negligible downward momentum transfer of upper-level flow (Whiteman and Doran 1993). In most mid-latitude regions, diurnal mountain winds are best observed during the summer months when climatological high pressure dominates.
This contribution focuses on the slope wind component of the diurnal mountain wind system. Slope winds are confined to flowing up or down the slopes of a valley wall or an isolated hill. Alternative terms for these winds are anabatic (upslope) and katabatic (downslope) winds. However, these terms refer to any flows that occur up or down the topography (Zardi and Whiteman 2012). Katabatic flows also commonly describe larger scale slope flows occurring on ice sheets such as Greenland or Antarctica (van den Broeke and van Lipzig 2003; Vihma et al. 2011). This specific type of katabatic flow is influenced by Coriolis accelerations (Shapiro and Fedorovich 2008) and has little to do with fire weather; thus it is not covered in detail here. This contribution will also not cover synoptically driven downslope windstorms, or foehn winds, which have important impacts on fire weather, fire behavior, and pollutant transport.
Driving Mechanisms of Slope Winds
Mass flux transported by slope flows is directly proportional to thermal forcing by available sensible heat and inversely proportional to static stability (Vergeiner and Dreiseitl 1987). This implies that slope flows are strongly constrained by the availability of radiative forcing and are determined by instantaneous equilibria (Vergeiner and Dreiseitl 1987). For instance, a disruption of slope winds occurs if clouds obscure the surface from incoming solar radiation.
Further, pressure gradients will be reduced in time as air flowing downslope (upslope) warms (cools) adiabatically. In the case of neutral or unstable atmospheres, upslope winds may not develop as air parcels will ascend vertically rather than moving along the slope (Orville 1964).
Theoretical and numerical models as well as observations indicate slope flows are nonsteady (Hunt et al. 2003; Fedorovich and Shapiro 2009; Zardi and Serafin 2015). Periodicity in temperature and wind speeds vary as a function of slope angle (Fleagle 1950), the temperature difference between the katabatic layer and the adjacent valley (McNider 1982), and horizontal advection and atmospheric stability (Zardi and Serafin 2015). Characteristic periods of slope flows are approximately 1.5 h (Doran and Horst 1981), with the period becoming shorter as stability increases (McNider 1982).
Differences Between Anabatic and Katabatic Winds
Anabatic winds tend to have higher velocities compared to nocturnal katabatic winds. This is because daytime radiative flux exchanges are greater, promoting more significant buoyancy perturbations. Through the interaction with valley wind systems and the transfer of valley air to slopes, anabatic winds will have deeper boundary layer penetration compared to katabatic winds, which are very shallow. Katabatic winds grow in depth as slope length increases with measured depths approximately 150 m above ground level (Whiteman and Zhong 2008). Typically, the height of the katabatic jet (velocity maxima) is a few meters above ground level, which places severe constraints on the ability of numerical weather prediction models to accurately simulate this phenomenon (Smith and Porte-Agel 2014).
Contribution of Katabatic Winds to Cold Pool Formation
Katabatic winds are also known as drainage flows. While small-scale cold air drainage flows and katabatic winds both result from density perturbations produced by radiative cooling, katabatic winds require a minimum slope and will also establish compensating currents (Lawrence 1954). Nonetheless, both katabatic winds and small-scale cold air drainage contribute to cold air pooling in mountain valleys and strengthen the formation of temperature inversions produced by localized radiative cooling (Fig. 2a; Smith and Skyllingstad 2005). Cold pool formation is important with regards to wildland and prescribed fires because of the impacts of smoke transport and smoke persistence on populated areas in valley bottoms (Goodrick et al. 2013) and reductions in fire activity (Sharples 2009). Katabatic winds traveling downslope spread out horizontally at the elevation where the buoyancy of the flow equals that of the stratified air in the valley (Stull 2017). Above the characteristic top of the inversion, a region of climatologically warmer minimum temperatures known as the thermal belt occurs (Fig. 2a). The warmer minimum temperatures result in lower relative humidity and greater evaporative demands that cause fuels to cure more quickly following precipitation. Due to the poor nocturnal moisture recovery and drier fuels, more active fire conditions are common along the thermal belt compared to adjacent terrain (Sharples 2009).
Environmental Influences on Slope Winds
The characteristics of diurnal mountain winds, such as their velocity, depth, duration, and timing, varies as a function of local terrain characteristics (i.e., inclination (slope) and orientation (aspect); Whiteman et al. 1989; Barry 2008; Fernando et al. 2015). Land surface characteristics such as soil moisture, vegetation, terrain orientation (aspect), as well as the underlying geology and geomorphology, influence both surface albedo and surface roughness. These surficial characteristics ultimately impact the surface energy budget and determine how air will flow up and down mountain slopes (Barry 2008). These factors may also have a seasonal dependence that affects the amplitude of horizontal pressure gradients that produce winds (Cogliati and Mazzeo 2006).
Slope flows quickly respond to variations in sensible heat flux that result from variations in the local energy budget. Clear sky conditions, which frequently coincide with anticyclonic upper level flow regimes, facilitate optimal longwave emission and surface cooling during the night and incoming solar radiation during the day. Other factors influencing net radiation on a slope include shading, elevation, soil moisture, the partitioning of latent and sensible heat fluxes, snow cover, and vegetation (Barry 2008). Correctly forecasting the timing and magnitude of slope flows in complex terrain will require improvements in both observations, data assimilation, and physical representation in numerical models of these processes (Fernando et al. 2015).
Interactions Between Slope, Valley, and Mountain-Plains Winds
Differentialheating of mountain valley sidewalls produces contrasts in instantaneous latent and sensible heat fluxes (Whiteman et al. 1989). This differential heating acts in concert with local slope flows to produce cross-valley, along-valley, and mountain-plain wind circulations (Whiteman 2000). The anabatic (katabatic) wind is the ascending (descending) component of the cross-valley circulation, which blows toward the valley sidewall experiencing peak heating. Along-valley circulations develop at night as cold descending air flows down the valley. Warm, ascending air flows up the valley during the day. The weak low pressure that results in the valley from the removal of air as it moves upslope facilitates the formation of an upstream flowing along-valley circulation. Weak, return flow aloft will develop, oriented down-valley during the daytime and up-valley during the nighttime; these are called anti-valley winds (Stull 2017). At the mountain range scale, the mountain-plain wind system is a larger version of diurnal slope winds (Koch et al. 2001). It originates due to thermal contrasts in the air over an entire mountain range and adjacent plain with winds blowing up (down) the mountain flank during the day (night). Elevated convergence induced by thermally-forced winds at all spatial scales can facilitate moist convection (Koch et al. 2001) leading to potential fire ignitions from lightning.
Applications to Fire Management
Correctly forecasting slope winds has practical applications for management of both wild and prescribed fire. Slope winds and cold air drainage can influence the distribution of air temperature in complex terrain, particularly through the formation of cold air pools. Cold pool formation, as well as the advection of temperature and moisture, influence nocturnal relative humidity recovery and fuel moisture with potential implications for fire behavior (Sharples 2009). Fires burn uphill burn faster and more intensely than those on flat slopes due to changes in flame dynamics and induced flow as well as flame proximity or contact with upslope vegetation that preheats fuels (Morandini et al. 2018). The magnitudes of slope winds (typically <5 m/s or 18 km/h) are sufficient to aid fire spread but unlikely to be the sole driver of extreme fire behavior. In the presence of stronger synoptic winds, slope flows could contribute to potential extreme fire behavior under conditions of highly receptive fuel beds. Suppression efforts must recognize the potential for rapid reversal of wind direction and thus fire growth during the morning and evening transition times (Sharples 2009; Fig. 3a). For example, a transition in wind direction could pose hazards to fire suppression personnel and values at risk if the direction of fire growth or ember-fall shifts toward unburned slopes or landscape features such as narrow canyons where flow channeling could occur (Whiteman and Doran 1993).
An important application of slope winds pertains to smoke transport and air pollution management, especially during prescribed fire operations. Under conditions of low fuel availability or connectivity, and/or high fuel moisture content, a fire may not attain sufficient energy release to create a convective column of air. In this scenario, surface winds, and not winds aloft, will bear the primary responsibility for smoke dispersal. During the smoldering phase of burning when a convective plume fails to develop, surface winds are the primary determinant of smoke transport (Whiteman 2000). Anabatic winds can blow smoke upslope and facilitate entrainment of smoke into the larger scale flow aloft, enabling long-range transport in the absence of a convective plume. Katabatic winds transport smoke downslope and promote smoke accumulation in valleys in conjunction with the cold air pool formation. This concentration of pollutants in valleys can lead to air quality concerns with regards to health and visibility (Goodrick et al. 2013). If the smoke impacts populated areas, it also may create public opposition to future prescribed burn operations (McCaffrey 2006). Careful assessment of meteorological conditions favoring or inhibiting slope winds during prescribed burns can help minimize smoke impacts on populated regions (Larkin et al. 2009) and estimate fire behavior (Sharples 2009) when wildland fires are ignited under quiescent synoptic flow regimes from dry lightning or human ignition sources. Smoke transport model assumptions need to be considered when slope winds are of concern. For instance, slope winds can violate assumptions made in simplistic Gaussian plume models (which assume steady-state, homogenous transport of pollutants), reducing their reliability (Goodrick et al. 2013). More advanced pollutant dispersion models are required if slope winds are of concern for smoke management, with improved results produced by coupled weather, dispersion, emissions, fuels, and consumption models such as BlueSky (Larkin et al. 2009).
A primary challenge in simulating slope winds results from the nearly infinite suite of potential configurations and spatial scales of terrain, geology, and vegetation in mountain environments. This limits the applicability of specific field measurements and numerical experiments to a broader understanding of the diurnal mountain wind system (Zardi and Whiteman 2012). Many numerical and analytical models have been developed and have yet to match observations, making selection of suitable models for varying circumstances difficult (Whiteman and Zhong 2008). Katabatic flows do not follow Monin-Obhukov scaling used in numerical weather prediction models (Grisogono et al. 2007). The vertical scales of motion in katabatic flows are particularly sensitive to model configuration and thus very difficult to accurately simulate (Smith and Porte-Agel 2014).
Slope winds are thermally driven winds occurring in areas of mountainous terrain and characterized by a morning and evening reversal of downslope (katabatic) and upslope (anabatic) flow. Slope winds most frequently occur during quiescent large-scale weather with clear sky conditions and are a component of the regional mountain valley-wind circulation. Although slope winds are generally shallow (near-surface) and of weak magnitude (<5 m/s or 18 km/h), they can influence fire behavior and play roles in the dispersal and transport of smoke. Fire managers are recommended to consider the timing and direction of slope winds when planning fire operations to mitigate the risks and impacts associated with wildland and prescribed fire.
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